DMC catalysts for the polyaddition of alkylene oxides to starter compounds containing active hydrogen atoms are known. See, for example, U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849 and 5,158,922. The use of these DMC catalysts for the preparation of polyether polyols reduces the content of mono-functional polyethers with terminal double bonds, the so-called “mono-ols”, in comparison to the preparation of polyether polyols with alkali catalysts, such as alkali metal hydroxides.
Polyether polyols produced by DMC catalysis can be used to process high-quality polyurethanes (e.g. elastomers, foams and coatings). DMC catalysts are usually obtained by reacting an aqueous solution of a metal salt with an aqueous solution of a metal cyanide salt in the presence of an organic complexing ligand, e.g., an ether. In a typical DMC catalyst preparation, aqueous solutions of zinc chloride (in excess) and potassium hexacyanocobaltate are mixed to form a dispersion. Dimethoxyethane (glyme) is then added to the dispersion. After filtering and washing the dispersion with the aqueous glyme solution, an active catalyst of the general formulaZn3[Co(CN)6]2.xZnCl2.yH2O.zglyme is obtained. See, for example, EP 700 949.
The following references disclose DMC catalysts which use tert-butanol as the organic complexing ligand (by itself or in combination with a polyether) in the preparation of polyether polyols to further reduce the content of mono-functional polyethers with terminal double bonds: JP 4145123; U.S. Pat. No. 5,470,813; EP 700 949; EP 743 093; EP 761 708; and WO 97/40086. Additionally, the use of these DMC catalysts reduces the induction time in the polyaddition reaction of alkylene oxides with corresponding starter compounds. Catalyst activity also increases. By shortening alkoxylation times, the process of preparing polyether polyols becomes more cost effective. Additionally, due to their increased activity, DMC catalysts can be used in low concentrations (25 ppm or less), making the expensive process of removing catalysts from polyether polyols no longer necessary.